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The figures below provide verification examples, over multiple or single cases, illustrating typical IFS model characteristics, be they strengths or weaknesses. Key points for the forecast user are highlighted in the captions.
Fig8.1.1016.3.1A-1: Reliability diagram of tropical cyclone strike probabilities, at day 10, for TCs in existence at T+0, for year July to June (years indicated by colours) showing reasonably good reliability (plots are near the diagonal), but a tendency towards over- confidence (plots to right of diagonal).
Fig8.1.1016.3.1B-2: Relative operating characteristics (ROC) diagram for tropical cyclone strike probabilities, at day 10, for TCs in existence at T+0, for year July to June (years indicated by colours) showing high ROC score (plots lie towards left and upper axes) with low False Alarm Rate and high Probability of Detection.
Fig8.1.1016.3-3.2: IFS model forecast location error of tropical cyclones (HRES ( and CTRL) ) - blue; Ensemble Mean (EM) - Yellow) and ENS spread (Red dots). The diagram indicates that in 2016 the average location error and spread at:
- Day3: HRES (and CTRL) location error about 180km; EM location error about 190km; ENS spread about 190km.
- Day5: HRES (and CTRL) location error about 340km; EM location error about 320km; ENS spread about 310km.
Fig8.1.1016.3.3-4: An illustration of what can happen when the model resolution is increased. HRES (and CTRL) model performance Aug-Nov 2015 using test runs (~9km resolution - Red; ~16km resolution - Blue), both without ocean coupling. On average, ~9km resolution forecast location error is slightly better to about Day5 but marginally worse from Day5 to Day7. However, beyond about Day5 the low sample size makes statistics unreliable and ~9km resolution is unlikely to be significantly different to ~16km resolution. Users should note that this diagram is included to illustrate that resolution changes have a significant impact.
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It should be remembered that the analysis process seeks to maintain dynamic and thermal balance within the IFS. To do this, it is possible that valid observations may be given low weight or be rejected near sharp upper troughs, or in the vicinity of deep active weather systems. Users should consider the potential for increased uncertainty in the subsequent evolution while bearing in mind the observations marked by increment vectors have, at least partially, been accepted and incline the analysis to better reflect the true structure of the atmosphere.
Fig8.1.1016.3.4-5: Tropical cyclone strike probability for Hurricane Florence from 12UTC 6 Sep 2018 (left). Large upper troposphere increments are evident near the location of the hurricane during data assimilation (centre) and the related modifications to the IFS model atmosphere will have had some impact upon the subsequent forecast run at 00UTC 7 Sep 2018 (right).
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The forecast central pressure and track of a tropical storm can show quite large run-to-run variations. The example below shows HRES and ENS forecasts from two runs ( data times 12UTC 30 July 2017 and 12UTC 1 August 2017) to illustrate the differences that can occur. In fact none of the HRES or ENS mean forecasts were correct. The weakening tropical storm actually moved northeast through central Japan. Outlying ENS members proved the better guidance in each case.
Fig8Fig8.1.1016.3.6A(Left)-6: Strike probabilities for NORU up to 12UTC 8 August 2017 (T+240) based on ENS and HRES forecast, data time 12UTC 30 July 2017.
Fig8Fig8.1.1016.3.6B(Right)-7: Strike probabilities for NORU up to 10 August 2017 (T+240) based on ENS and HRES forecast, data time 12UTC 1 Aug 2017.
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- The earlier forecast has a 30% threat to coastal central eastern Japan and Tokyo, 10%-20% threat to the islands south of Kyushu, but little threat to the Sea of Japan.
- The later forecast has <10% threat to eastern Japan and Tokyo but 30%-40% threat to the Sea of Japan (and 40%-50% in the south), and 70%-80% threat to the islands south of Kyushu.
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Extra-tropical Transition
Extratropical transition Extra-tropical transition of tropical storms is, in general, difficult for NWP models to deal with. NWP models often show large run-to-run variability in forecast track, movement and depth of a tropical depression as it moves into the mid-latitudes. Small differences between the analyses and background fields in sequential runs, or small perturbations developed by NWP models in the circulating flow, can have a major impact on the forecast evolution. This is especially so with energetic systems, such as well developed tropical storms, and can make some forecast aspects unsafe later in the forecast period (say beyond Day5).
Fig8Fig8.1.1016.3.7A(Left):-8: Strike probabilities for tropical cyclone17W up to 10 August 2017 (T+240) based on ENS and HRES forecasts, data time 12UTC 1 August 2017. It is notable that the ENS mean track (dotted line) and the tracks of the majority of ENS members veer eastwards towards mid-Pacific as extratropical extra-tropical transition occurs while HRES (solid line) and a few ENS members curve the low pressure centre westwards towards Japan.
Fig8Fig8.1.1016.3.7B(Right): -9: The same case as in Fig8.1.1016.3.7A-7. Top: Probability from ENS members for tropical cyclone 17W to fall into each of the 5 tropical cyclone intensity categories shown at 6hr intervals to 10 days. Centre and Bottom: Lagrangian meteograms of distribution of the ENS for the 10m wind (kt) and MSLP (hPa) at tropical cyclone 17W centre. Tropical cyclone 17W is considered to have become extratropical extra-tropical by Day9 and central pressure and wind information is discontinued at that time. HRES central pressure and winds diverge from the ENS beyond Day5 and forecasts cease as winds fall below the threshold for a tropical cyclone to exist and the depression effectively transitions to become extratropical extra-tropical (or even non-existent) sooner than it appears to in most (~80%, see top row) of the ENS members.
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It is inadvisable to use strike probability values right at the edge of the domain, because there is no outside-domain detection of cyclones. The reduced resolution that is used (~45km) may be one reason.
Fig8Fig8.1.1016.3.8-10: Strike probabilities of NORU up to 17 August 2017 (T+240) based on ENS and HRES forecast, data time 00UTC 7 August 2017. ENS mean track (dotted line) and HRES track (solid line). Crosses mark previous positions of NORU. The strike threat area is shifted to the northwest of the forecast track of the centre shown by the ENS mean (dotted line), and the HRES (solid line), whereas the strike threat should be approximately centred on the ENS mean track.
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Rarely two Tropical Storms are relatively close the possibility arises that the tracking algorithm may jump from one to another - e.g. if one is decaying. This seems to be a very rare occurrence and which is under investigation, but users should be aware of the potential for this error.
Fig8Fig8.1.1016.3.9-11: Precipitation and surface isobaric forecast NW Australia and NE Indian Ocean T+156 VT12UTC 29 Feb 2020. TC Ferdinand is moving slowly off NW Australia and weakening. TC Esther is moving towards the SW and strengthening.
Fig8Fig8.1.1016.3.10-12: Locations of TC Ferdinand from HRES forecast DT 00UTC 23 Feb 2020 (solid line on chart). At 12UTC 29 Feb 2020 the algorithm incorrectly identifies the location of Ferdinand at the location of TC Esther and subsequently follows the movement and developments of TC Esther. The sudden change in the HRES forecast values of central pressure (much lower) and 10m wind (much higher) are shown on the respective graphs - these correspond to TC Esther. On this occasion the forecast values of ENS mean and ENS probabilities do not suffer from the same problem.
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A limit is placed on the roughness length scale parameter in order to avoid the effect of too much drag from the sea surface in the lower atmosphere and enable more realistic (stronger) winds to be forecast in the vicinity of relatively intense tropical cyclones. It should also be noted that tropical cyclone development with strengthened winds has only a limited effect upon the size and character of waves developed by the Wave model. The model change to limit the roughness length scale at very high speeds was introduced in cycle 47r1 in June 2020. See Section on Waves near tropical storms.
RSMC official forecasts of tropical cyclones take precedence
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